1. Field of the Invention
The present invention generally relates to optical amplifiers, and more specifically relates to rare earth doped fiber amplifiers for amplifying optical signals by providing a pump signal.
2. Description of the Related Art
In general, rare earth doped fiber amplifiers such as erbium doped fiber amplifiers have been used for optical communication systems. In erbium doped fiber amplifiers, it is well known that an amplified optical signal can be obtained by providing to the erbium doped fiber a pump signal having a shorter wavelength than that of the optical signal to be transmitted. Furthermore, the pump power of the pump signal can be adjusted automatically by monitoring an amplified spontaneous emission (ASE) signal. However, when monitoring the forward propagating ASE signal, conventional gain control methods require a complicated extracting means, such as Fiber Bragg Gratings (FBG) or Array Waveguide Gratings (AWG) to extract the forward ASE signal from the amplified optical signal. On the other hand, when monitoring the backward propagating ASE signal, since a backward ASE signal with enough power can not be obtained, complicated control techniques are needed to control the pump power based on such weak backward ASE powers.
As a result, conventional erbium doped fiber amplifiers each provided with a gain controller for performing the conventional gain control methods mentioned above would become larger and more complicate as the number of channels in a density multiplexed input signal increase. It is thus desired to establish a simple gain controller for controlling the gain of rare earth doped fiber amplifiers.
It is an object of the present invention to provide an improved optical amplifier with a simple gain controller.
It is another object of the present invention to provide an improved rare earth doped fiber amplifier with a gain controller for controlling the gain of a pump signal and a monitor coupler for monitoring an amplified spontaneous emission (ASE) signal.
In order to achieve the above object of the present invention, an optical amplifier for amplifying an optical signal, wherein the amplifier comprises:
an input port for inputting the optical signal to be amplified,
a rare earth doped optical fiber, coupled to said input port, for amplifying the optical signal,
a first pump laser, coupled to said rare earth doped optical fiber, for providing a first pump signal to said rare earth doped optical fiber in the direction of the optical signal,
a wavelength division multiplexer, coupled between said input port and said rare earth doped optical fiber, for extracting a backward traveling amplified spontaneous emission signal in a wavelength band different from a wavelength band containing the optical signal,
a gain controller for controlling a power level of the first pump signal based on a received power of the backward traveling amplified spontaneous emission signal.